1. Field of the Invention
The present invention relates to air press assemblies of papermaking machines, and, more particularly, to roll configurations used in such air press assemblies.
2. Description of the Related Art
For many years attempts have been made to use external air pressure to force water out of a paper web. Rather than compress a sheet at a press nip to the point where hydraulic pressure drives water out, as is the case in normal wet pressing, it was reasoned that more water could be removed, and sheet bulk could be maintained, if air pressure could be applied to supplement roll nip generated hydraulic pressures. One such attempt involves providing a multi-roll or other structure forming an air press having a closed chamber.
One significant challenge that exists in designing an effective multi-roll air press assembly is in minimizing the leakage of air from the assembly adjacent each set of roll ends, with each set of roll ends at least partially defining a chamber end. Typically, a respective seal assembly is held in tension against each chamber end. However, the opportunity for air leakage from the nip region between respective rolls exists, especially in the end portions of each roll not in contact with the paper web and at least one web transfer fabric being fed therethrough.
Another challenge that exists in designing an effective multi-roll air press assembly is in minimizing the cost, labor and down time associated with the replacement of a worn roll, especially large-diameter main rolls. Even if some type of a surface cover is used in such an instance, replacement involves removing the surface cover from the entire roll length and then placing on a new surface cover. The expense can be increased if the entire roll should instead require replacement.
What is needed in the art is a roll for use in an air press assembly of a papermaking machine that results in decreased air leakage from multi-roll air presses adjacent nip ends thereof, and has readily accessible and replaceable portions which tend to wear for a certain amount of time before any significant wear begins to occur on the primary part of the roll.